A need exists for a process that will enable selective oxidation of organic compounds to alcohols and olefins. However, stopping the oxidation of organic compounds at alcohols and olefins is difficult. For example, the ease of oxidation of carbon-hydrogen bonds increases in the order of, HC&lt;H--C--OH&lt;H--C.dbd.O&lt;H--C(.dbd.O)OH. For this reason, oxidation reactions often give a complex array of extensively oxidized ketone and carboxylic acid products. Under some conditions, the oxidation reaction proceeds all the way to CO.sub.2.
One long desired goal has been to provide an economical and controllable oxidation process whereby methane is converted into desired higher organic chemicals. This is due, in part, to the large and stable raw material supply of natural gas, with reserves estimated to be about 2180 trillion cubic feet in 1986. Few technologies have emerged, however, which are viable for direct methane conversion. An example of such technologies includes partial flame oxidation, electric-arc processes, oxidative couplings, and the Benson process. These methods typically involve very high temperatures or very aggressive reactants which provide poor selectivity and less than desirable efficiency. The prominent barrier to methane conversion is the strong C--H bond strength of 104 kcal/mol. The methane molecule is largely inert or, once enough energy is applied to cleave a C--H bond, continuing reactions are rapid and nonselective, with combustion being a prime example. The challenge in methane utilization is to carry out functionalization in a controlled, selective fashion so that valuable intermediate products, such as methanol, can be obtained.
Methane is not the only organic compound feed stock of which selective oxidation would be desired. For example, the oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone is an important industrial process. The resultant ketone/alcohol mixture is used in the production of adipic acid, an important reagent in the synthesis of nylon 66. In the United States, 98% of the adipic acid produced is made by ketone/alcohol mixture oxidation and 90% of the acid is used for nylon manufacture. Ketone/alcohol mixture is also used in the production of .epsilon.-caprolactam, used in the manufacture of nylon 6. It would be desirable to develop a process for oxidation of cyclohexane which is substantially selective to cyclohexanol.
The conventional process of cyclohexane oxidation utilizes a cobalt catalyst and air as an oxidant. Typical conditions are 150.degree.-160.degree. C. and 8-10 atmospheres. To minimize over-oxidation, conversion per pass is limited to 4-6 mol percent. This requires that cyclohexane be distilled from the product mixture before recycling. Selectivity to the ketone/alcohol mixture is about 75-80%.
It is a principal objective of this invention to develop a process which is capable of oxidizing a wide range of organic compounds controllably to selected end products.